Reliability is a term a lot of people use without fully understanding what it means, much like service and dependability. When using reliability within calculations, you are using the probability that a component will perform its required function under the stated conditions for a specific period of time and is able to provide an accurate result or outcome during that time.
The method used to assess the reliability of a component is called Mean Time Between Failure (MTBF), a measure based on statistical estimates of the average time it should run before requiring repair. Although MTBF is a measure of reliability, it should not be confused with the fact that a product with a high MTBF will last as long as a similar product with a low MTBF.[i]
Mean Time Between Failure
For example, if the MTBF of a component is 40,000 hours, it doesn’t mean that the power supply will last for 40,000 hours, it means that the statistical average then becomes the true average as the number of samples increase:
The formula to measure the MTBF is:
MTBF = Total time/Number of failures[ii]
A similar measure that sometimes confuses the issue is Mean Time To Failure (MTTF). MTTF is a statistical value for the expected failure time of a component and CANNOT be repaired, whereas, MTBF is the key statistical value for systems that CAN be repaired and resume service.[iii]
A common way to show MTTF is the ‘Bathtub Curve’: [iv]
Mean Time To Failure "Bathtub Curve"
This shows a component’s MTTF has a high probability of failure within the first few hours or weeks of operation and then it comes down and stays relatively low for an extended period of time, increasing sharply when its normal life has run out.
The formula for MTTF, to clarify more distinctly between MTBF, is understood that the total number of hours of service is divided by the number of components:
MTTF = Total time/Number of units under test [v]
*It is only when all parts fail with the same failure mode that MTBF converges to MTTF.[vi]
Before we take a look at MTBF for printers and copiers, how long do you think your printers’ MTBF is?
